What is the importance of the color and number of door nails in ancient gates?

"The Huidian of the Qing Dynasty" records: "The palace gates and verandahs are all covered with yellow sulfur glass, and the doors are equipped with gold nails." There are clear regulations on the number of doornails used in the palaces of princes, princes, princes, etc.: "Prince's mansion system, the main entrance has five bays, and the doornails are nine bays vertically and seven horizontally." Prince seven-two (minus two-sevenths)"; "The county prince, Baylor, Beizi, Zhenguo Gong, and Fuguo Gong are the same as the Prince's Mansion"; "The nails on the public gate are seven in length and width, and the number is reduced to five-five from the prince to the male. , are all made of iron." Ordinary people cannot use door nails at all.